This invention relates to PEM fuel cells, and more particularly to corrosion-resistant electrical contact elements therefor.
Fuel cells have been proposed as a power source for electric vehicles and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called xe2x80x9cmembrane-electrode-assemblyxe2x80x9d comprising a thin, solid polymer membrane-electrolyte having an anode on one face of the membrane-electrolyte and a cathode on the opposite face of the membrane-electrolyte. The anode and cathode typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles. One such membrane-electrode-assembly and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of the present invention. The membrane-electrode-assembly is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, and may contain appropriate channels and openings therein for distributing the fuel cell""s gaseous reactants (i.e., H2 and O2/air) over the surfaces of the respective anode and cathode.
Bipolar PEM fuel cells comprise a plurality of the membrane-electrode-assemblies stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or septum. The septum or bipolar plate has two working faces, one confronting the anode of one cell and the other confronting the cathode on the next adjacent cell in the stack, and electrically conducts current between the adjacent cells. Contact elements at the ends of the stack contact only the end cells and are referred to as end plates.
In an H2-O2/air PEM fuel cell environment, the bipolar plates and other contact elements (e.g., end plates) are in constant contact with highly acidic solutions (pH 3-5) containing Fxe2x88x92, SO4xe2x88x92xe2x88x92, SO3xe2x88x92, HSO4xe2x88x92, CO3xe2x88x92xe2x88x92, and HCO3xe2x88x92, etc. Moreover, the cathode operates in a highly oxidizing environment, being polarized to a maximum of about +1 V (vs. the normal hydrogen electrode) while being exposed to pressurized air. Finally, the anode is constantly exposed to super atmospheric hydrogen. Hence, contact elements made from metal must be resistant to acids, oxidation, and hydrogen embrittlement in the fuel cell environment. As few metals exist that meet this criteria, contact elements have often been fabricated from large pieces of graphite which is corrosion-resistant, and electrically conductive in the PEM fuel cell environment. However, graphite is quite fragile, and quite porous making it extremely difficult to make very thin gas impervious plates therefrom.
Lightweight metals such as aluminum and titanium and their alloys have also been proposed for use in making fuel cell contact elements. Such metals are more conductive than graphite, and can be formed into very thin plates. Unfortunately, such light weight metals are susceptible to corrosion in the hostile PEM fuel cell environment, and contact elements made therefrom either dissolve (e.g., in the case of aluminum), or form highly electronically resistive, passivating oxide films on their surface (e.g., in the case of titanium or stainless steel) that increases the internal resistance of the fuel cell and reduces its performance. To address this problem it has been proposed to coat the lightweight metal contact elements with a layer of metal or metal compound which is both electrically conductive and corrosion resistant to thereby protect the underlying metal. See for example, Li et al U.S. Pat. No. 5,624,769, which is assigned to the assignee of the present invention, and discloses a light metal core, a stainless steel passivating layer atop the core, and a layer of titanium nitride (TiN) atop the stainless steel layer.
The present invention comprehends a PEM fuel cell having at least one cell comprising a pair of opposite polarity electrodes, a membrane electrolyte interjacent the electrodes for conducting ions therebetween, and an electrically conductive contact element confronting at least one of the electrodes. The contact element has a working face that serves to conduct electrical current from that electrode. The contact element comprises a corrosion-susceptible metal substrate, having an electrically conductive, corrosion-resistant, protective polymer coating on the working face to protect the substrate from the corrosive environment of the fuel cell. By xe2x80x9ccorrosion susceptible metalxe2x80x9d is meant a metal that is either dissolved by, or oxidized/passivated by, the cell""s environment. An oxidizable metal layer may cover a dissolvable metal substrate, and underlie the conductive polymer layer.
More specifically, the protective coatings of the present invention comprises a plurality of electrically conductive, corrosion-proof (i.e., oxidation-resistant and acid-resistant) filler particles dispersed throughout a matrix of an acid-resistant, water-insoluble, oxidation resistant polymer that binds the particles together and holds them on the surface of the metal substrate. The coating contains sufficient filler particles to produce a resistivity no greater than about 50 ohm-cm, and has a thickness between about 5 microns and about 75 microns depending on the composition, resistivity and integrity of the coating. Thinner coatings (i.e., about 15-25 microns) are preferred for minimizing the IR drop through the stack. Impervious protective coatings are used directly on metals that are dissolvable by the system acids. Pervious coatings may be used on metals that are only oxidized/passivated, or on dissolvable metals covered with a layer of oxidizable/passivatable metal.
Preferably, the conductive particles comprise carbon or graphite having a particle size less than about 50 microns. Most preferably, the particles comprise a mixture of graphite with smaller carbon black particles (i.e., about 0.5-1.5 microns) that fill the interstices between larger graphite particles (i.e., about 5-20 microns) to optimize the packing density of said particles for improved conductivity. Other oxidation-resistant and acid-resistant conductive particles may be substituted for the small carbon black particles. The polymer matrix comprises any water-insoluble polymer that (1) is resistant to acids and oxidation, (2) can be readily coated or formed into thin films, and (3) can withstand the operating temperatures of the fuel cell (i.e. up to about 120xc2x0 C).
The coating may be applied in a variety of ways including: (1) laminating a preformed discrete film of the coating material onto the working face(s) of the conductive element; or (2) applying (e.g. spraying, brushing, doctor blading etc.) a precursor layer of the coating material (i.e. a slurry of conductive particles in solvated polymer) to the working face followed by drying and curing the film, or (3) electrophoretically depositing the coating onto the working face(s).